The invention relates to an optical scanning device for scanning, in a first mode of operation, a first type of record carrier having a first information layer and a first transparent layer of a first thickness and for scanning, in a second mode of operation, a second type of record carrier having a second information layer and a second transparent layer of a second thickness, different from the first thickness, which device comprises a first radiation source for generating a first, HD, radiation beam in the first mode and a second radiation source for generating a second, LD, radiation beam in the second mode, an objective system designed for operation at a first set of conjugates to focus the HD beam on the first information layer in the first mode and for operation at a second, different, set of conjugates to focus the LD beam on the second information layer in the second mode wherein a beam vergence-changing lens is arranged in the path of at least one of the LD and HD beams.
The HD beam and the LD beam are herein understood to mean the beam used for scanning an information layer with a higher information density and an information layer with a lower information density, respectively.
A beam vergence-changing lens is understood to mean an auxiliary lens arranged in the path of a beam between the radiation source and the conventional lens system of a scanning device, which lens adapts the divergence or convergence of the beam in such a way that this beam has the required cross-section at the entrance pupil of the conventional lens system.
U.S. Pat. No. 4,823,334 discloses an optical scanning device wherein a collimating means for converting a divergent beam from a radiation source into a collimated beam incident on the objective system comprises a first lens element and a second lens element. The second lens element has a weak power so that collimating adjustment can be carried out with extreme accuracy by moving this element. This scanning device is intended to read/write a low-density record carrier and comprises only one radiation source which emits only one beam having one wavelength.
Generally, the transparent layer in optical record carriers is intended to protect the information layer from ambient influences, keeping dust particles, scratches etc at a sufficient distance from the information layer and it may provide mechanical support to the information layer. In the latter case, the transparent layer functions as a substrate for the information layer and the thickness of the transparent layer is a compromise between the thickness which is desired to give the record carrier the desired rigidity and the thickness which is desired in connection with the numerical aperture (NA) of the scanning beam incident on the transparent layer.
The NA of the objective system on the side of the record carrier is determined by the resolution the scanning device must have to read or write an information layer. The resolution of the scanning device, which resolution is inversely proportional to the minimum scanning spot size that can be formed by the device, is proportional to NA/xcex, wherein xcex is the wavelength of the scanning beam. For scanning a record carrier with a larger information density, like the DVD, a scanning beam, hereinafter a HD (high density) scanning beam, should be used which has a higher NA and a smaller xcex than the scanning beam, hereinafter a LD (low density) scanning beam, used for scanning a record carrier with a lower information density. For record carriers with a larger information density, thus requiring a higher NA, it is often necessary to reduce the thickness of the information layer so as to decrease the influence of tilt of the record carrier with respect to the optical axis of the scanning device on the quality of the focus, or scanning spot. With the advent of novel record carriers having larger information densities, different types of record carriers having different thicknesses of the transparent layer will be on the market. A compatible scanning device will have to be able to scan the different types of record carriers, independently of the thickness of the transparent layer. Such a scanning device comprises a first and a second radiation source, usually diode lasers, for generating the HD beam having a wavelength of, for example 650 nm, and for generating the LD beam having a wavelength of, for example 780 nm. The objective system of a compatible scanning device for two types of record carriers should have a first set of conjugates for scanning the first type of record carrier and a second, different, set of conjugates for scanning the second type of record carrier. The first conjugate of an objective system is herein understood to mean the distance between the object plane, i.e. the emitting surface of the radiation source, and the first principal plane of the objective system. The second conjugate of the objective system is herein understood to mean the distance between the second principal plane of the objective system and the image plane, i.e. the plane of the information layer. Scanning a record carrier is herein understood to mean moving a scanning spot, formed by a scanning beam, and the information layer relative to each other for the purpose of reading, writing and/or erasing information.
In order to obtain two scanning beams having different NAs with one objective system in a compatible scanning device, an annular dichroic filter or diffraction element may be arranged in the radiation path before the objective system or on the first surface of this objective system. Such a dichroic filter or diffraction element transmits the HD scanning beam and blocks or diffracts the rim of the LD scanning beam, so that only the central part of the latter beam is passed through the objective system to the LD information layer. The LD scanning beam forms a scanning spot on the LD information layer, which scanning spot is broader than that formed by the HD scanning beam on the HD information layer. A better alternative, especially for a compatible scanning device wherein the LD scanning beam is used not only for reading, but also for recording an information layer, and wherein a maximal quantity of radiation from the radiation source should reach the information layer, is to arrange an additional lens in the LD beam path before the objective system. Such a beam vergence-changing lens, for example a pre-collimator lens, changes the vergence of the beam from the source in such a way that the LD beam fills only the central part of the objective system and the NA of this beam is such that, after passage through the objective system, the beam has the required image side NA.
The pre-collimator lens in known scanning devices is a glass lens. Such a lens is relatively expensive as compared with the other lenses of the scanning device, for example the objective lens which is made of plastics. Although a plastics pre-collimator lens would be preferred for cost reasons, this has not been implemented in known scanning devices, because the focal length of such a lens varies with temperature changes. This variation of the focal length results in a shift of the focus of the LD beam relative to the LD information layer. Since the pre-collimator lens is arranged in the path of the LD beam from the radiation source to the record carrier only, and not also in the path of the reflected LD beam from the record carrier to the detection system, this focus shift is interpreted by the focus detection system of the scanning apparatus as a shift of the objective system and the plane of the momentarily scanned part of the information layer relative to each other. The focus servosystem then moves the objective system and the information layer relative to each other, such that the focus error signal becomes zero, which results in a focus offset.
It is an object of the invention to provide an optical scanning device as described in the opening paragraph wherein the above-mentioned problems have been substantially reduced. This scanning device is characterized in that the beam vergence-changing lens is a plastic lens and has a focal length of between 6 and 9 mm.
The invention is based on the recognition that the defocusing due to temperature variations of a plastic lens is proportional to the design focal length of this lens and that for a design focal length of the order of 6 to 9 mm the remaining defocusing in the record carrier is fully acceptable. By a proper design of the device resulting in such a small focal length for the pre-collimator lens, it becomes possible to use a pre-collimator lens made of plastics with its costs advantage.
A first embodiment of the scanning device, wherein a beam vergence-changing lens is arranged in the path of the LD beam, is characterized in that this lens has a positive lens power.
This embodiment starts from a conventional scanning device wherein the HD beam has the proper vergence and fills the whole pupil of the conventional lens system. By means of the beam vergence-changing lens in the LD beam path, it is realized that this beam fills only the central part of said pupil.
A second embodiment of the scanning device is characterized in that a beam vergence-changing lens is arranged in the path of the HD beam, which lens has a negative lens power.
This embodiment starts from a scanning device wherein the LD beam has the proper vergence and fills the central part of the pupil of the conventional lens system. By means of the vergence-changing lens in the HD beam path, it is realized that this beam fills the whole pupil of said lens system.
The scanning device may also comprise a first beam vergence changing lens, having a positive lens power, in the LD beam and a second beam vergence-changing lens, having a negative lens power, in the HD beam.
The use of a beam vergence-changing lens with said small focal length results in a limited image field, which may be sufficient if the mutual positions of the optical elements can be controlled accurately. A scanning device, wherein larger position tolerances are required, is further characterized in that the beam vergence-changing lens has a concave entrance surface and a convex exit surface.
The entrance surface of the pre-collimator lens is the surface facing the second radiation source. Use of a concave/convex pre-collimator lens results in a larger image field, so that the requirements for the mutual positions of the optical elements can be lessened.
An embodiment of the optical scanning device, wherein the path of the beam provided with the beam vergence-changing lens comprises a beam-dividing diffraction element, is preferably further characterized in that the beam-vergence-changing lens is arranged between this diffraction element and the relevant radiation source.
The diffraction element splits the beam from the radiation source into a main beam, i.e. the scanning beam, and two secondary beams for tracking purposes. By arranging the pre-collimator lens before the diffraction element, instead of behind this element, this lens can be positioned sufficiently close to the radiation source, which is preferred in view of the short focal length. Moreover, it is better ensured that the secondary beams fall within the entrance pupil of the objective system.
The scanning device may be further characterized in that in the path of at least one of the LD and HD beams a beam shaper is arranged as a first element facing the relevant radiation source.
With such a beam shaper, the diode laser beam having an elliptical cross-section can be converted in a beam having a circular cross-section, without loss of radiation. An effective and small beam shaper, which can be arranged close to a diode laser, is disclosed in U.S. Pat. No. 5,467,335. By providing a beam shaper in the compatible scanning device, the intensity of the LD beam is increased, so that this beam is better suitable for writing information.